For an increasing amount of services in communication networks, wireless communication devices are communicatively connected to more than one radio base station. For instance, when determining a geographic location of a wireless communication device, reference signals from multiple radio base stations are used.
In communication networks which applies LTE (Long Term Evolution) based radio access technologies, accurate time and phase alignment of the internal clock is important. Time and phase synchronization is required for LTE-TDD (Time Division Duplex) many LTE-FDD (Frequency Division Duplex) coordination features e.g. for joint transmission, a wireless communication device receives data from multiple cells or multiple RBSs (Radio Base Stations), which offers better performance, but puts harder requirements on synchronization. In packet synchronisation networks, a major problem for synchronization protocols is the variance in the send time, access time, propagation time, and the receive time.
From the infrastructure perspective, mobile operators have a broad range of topologies to support. The physical network using different technologies such as microwave, fibre and copper wire will enable/limit different capabilities and characteristics. These differences in physical transport and in the different types of topologies, creates delay and delay variation that is unpredictable.
One solution for synchronising internal clocks in communication network is to distribute PTP (Precision Time Protocol) messages from a Grandmaster entity, which in generally is located centralised in the communication network, to PTP-clients at each cell site. The PTP protocol distributes PTP messages from a Grandmaster entity to transport network nodes and access network nodes who update their internal clocks based on the received time information in order to stay synchronized. A PTP system is a distributed, networked system consisting of a combination of PTP and non-PTP devices. PTP systems include a grandmaster entity, boundary clocks, and transparent clocks. Often the Grandmaster entity is located in a centralized part of the network; causing PTP messages to travel multiple hops. A boundary clock has multiple network connections and can accurately bridge synchronization from one network segment to another. A synchronization master is selected for each of the network segments in the system. The root timing reference is called the Grandmaster clock. The Grandmaster entity transmits synchronization information to the clocks that are in its network segment. The boundary clocks with a presence on that segment then relay accurate time to the other segments to which they are equally connected. The transparent clock modifies PTP messages by including appropriate timestamps as they pass through the device. The Timestamps in the PTP messages are compensated for time spent traversing the network and equipment e.g. (switch/router).
The term “wireless communication device” will be used throughout this description to denote any device which is capable of wireless communications. The term wireless communication device may thus include any device, which may be used by a user for wireless communications. Accordingly, the term wireless communication device may alternatively be referred to as a mobile terminal, a terminal, a user terminal (UT), a user equipment (UE), a wireless terminal, a wireless communication device, a wireless transmit/receive unit (WTRU), a mobile phone, a cell phone, a table computer, a smart phone, etc. Yet further, the term wireless communication device includes MTC (Machine Type Communication) devices, which do not necessarily involve human interaction. MTC devices are sometimes referred to as Machine-to-Machine (M2M) devices.
With reference to FIG. 1, which is a schematic overview, a scenario where a wireless communication device 100 is served by multiple access network nodes 102, 104 will now be described.
In this example the communication network node is communicatively connected via an LTE (Long Term Evolution) access network to a first radio base station 102 and a second radio base station 104. In the figure respective coverage areas of the radio base stations 102, 104 are illustrated as two ovals. The first radio base station 102 is set to be a master for a coordination function such as FeICIC (Further enhanced Inter-Cell Interference Coordination) for a specific wireless communication device, and controls some functionality of the second radio base station 104 which is set to be a slave. When the wireless communication device 100 is connected to multiple radio base stations 102, 104, for some functionality, the radio base stations need to fulfil some time synchronisation requirements. Such time synchronisation requirements are also known as phase synchronisation requirements. For instance, radio base stations could be time synchronised in accordance with the PTP (Precision Time Protocol). A centrally arranged Grandmaster entity distributes PTP packets according to PTP IEEE (Institute of Electrical and Electronics Engineers) 1588v2, end to end, to PTP-clients, e.g. the radio base stations.
In this example the wireless communication device 100 is located in an area between the dash-dotted lines A and B, where the wireless communication device 100 will be served by the radio base stations 102, 104. This area will be centred in the middle between the radio base stations 102, 104. In order for functions to work properly radio base station 104 coverage must at least partly be within the area between the dotted lines A and B. The PTP synchronised signals from the master and slave radio base stations 102, 104 which reach the wireless communication device may not differ more than +/−750 ns from each other, giving a total of 1500 ns, which allows for a distance between A and B of about 450 meters. However, due to the time synchronisation requirements, wireless communication devices 100 may be located within coverage of two radio base stations 102, 104 but outside the lines A and B and could therefore not be properly served, which is a problem. A better time synchronization precision in the transport network part would allow for an increased difference between the radio signals from radio base stations 102, 104 to the wireless communication device 100 and thereby an increased distance between the dotted lines A and B. This increased area between dotted lines A and B will then cover further wireless communication devices 100
Thus, there is need for a more effective use of installed communication resources in communication networks.